High-pressure phase transitions in Ca0.2Sr0.8Al2Si2O8feldspar

Abstract

A synthetic feldspar of composition Ca 0.2 Sr 0.8 Al 2 Si 2 O 8 (An 20 SrF 80 ) (space group I 1 at room condition, Q od = 0.88) was investigated using in situ high-pressure single-crystal X-ray diffraction. Unit-cell parameters were measured as a function of pressure to 7.7 GPa at room T . Two reversible phase transitions were observed in the investigated pressure range. A first-order triclinic I 1 to monoclinic I 2/ c phase transition was observed at P ~ 4.3 GPa. Several cycles through the transformation were made to constrain the hysteresis (~0.3 GPa). The discontinuous character of the transition is especially noticeable in the behavior of the γ angle, which increases from 90.55° to 91.04° before P tr . The Murnaghan EoS parameters obtained for the volume are: V 0 = 1393.97(6) A 3 , K 0 = 88.7(5) GPa, K ′ = 1.3(3) for the triclinic I 1 phase and V 0 = 1396.9(1) A 3 , K 0 = 89.4(9) GPa, K ′ = −1.7(6) for the monoclinic I 2/ c phase whose high- P behavior appears, therefore, linear in the pressure range of its stability. The axial compressibility scheme is β a > β c > β b for both phases, in good agreement with previous observations for other feldspars. A similar displacive ferroelastic I 1- I 2/ c phase transition was observed for the same sample with increasing temperature. At P ~ 7.3 GPa a large discontinuity in the unit-cell parameters indicates a further transition from monoclinic I 2/ c phase to monoclinic-II phase. This transformation is strongly fist order with a volume change of about 1.7%.